Hydrotalcite which is a kind of naturally-occurring layered clay mineral, is mainly comprised of hydroxides of elements that exist naturally abundantly, such as magnesium and aluminum, and the synthesis thereof is capable of being performed comparatively easily. Accordingly, various synthesizing methods have heretofore been disclosed.
For example, a patent document 1 discloses a method of producing a hydrotalcite in a water solvent, using magnesium hydroxide as a source of magnesium, while a patent document 2 discloses a method of allowing magnesium ion to react with aluminum ion in a solution under the presence of alkali.
Moreover, it has been known that a hydrotalcite has an anion exchange property, and thus it is expected that a hydrotalcite will be able to make a contribution to: water quality improvement of contaminated water; elution prevention of hazardous substances; improvement of soil quality; and promotion of stabilization of hazardous substances in a waste repository etc., in the fields of the safety improvement techniques of wastes and the detoxification and environment improvement techniques, if immobilization of anionic hazardous substances such as arsenic, fluoride, boron, selenium, sexavalent chrome, nitrite ion, and etc. is possible by taking advantage of such anion exchange property.
According to the conventional highly crystalline hydrotalcite products, however, due to the ion exchange occurring preferentially between the hydrotalcite products and carbon dioxide in the air or carbonate ions in water, ion exchange against target anions would not occur by an ordinary method. As a result, the conventional ones could not achieve the desired effect in immobilization of anionic hazardous substances. This is presumably because the highly crystalline hydrotalcite produced by the conventional techniques have such a large crystallite size that the ion exchanging capacity has been deteriorated.
Patent document 1
Japanese Un-examined Patent Publication No. 6-329410
Patent document 2
Japanese Un-examined Patent Publication No. 2003-26418